Recently I received the following question: Is the output of the HMC942LP4E multiplier AC or DC coupled?
A review of the datasheet reveals that the interface schematics for pin 4 (RFIN) and pin 15 (RFOUT) are identical; both have a shunt inductor to ground just beyond the pin. The pin descriptions however, state that the input is DC coupled while the description for the output states that it is AC coupled. This is a typographical error in the datasheet. Both the input and output are DC coupled. Often when the datasheet mentions DC coupling it implies that internally there is a DC path from the RFIN pin to the input of the amplifier or from RFOUT pin to the output of the output amplifier. In this case the shunt inductors that provide ESD protection also allow a DC path to ground. Any DC voltage placed at the input or the output will be shorted to ground through these inductors and most likely result in damage to the device.
Users that have circuitry that results in a DC voltage at the input or output nodes of the HMC942 multiplier will need to add a series capacitor (COG / NPO preferred) to prevent the flow of DC current into the multiplier. Ideally a reactance of 3 ohms or less for the highest output frequency intended to be used should be used bearing in mind the effects of the self resonant frequency of the capacitor, board parasitics and voltage rating of the capacitor. ATC's 600L series with their high self-resonant frequency (Fsrf) or 560L Ultra Broadband capacitors should work well for this application.
Evaluation board users that will have DC at the input or output of the HMC942 will need to use a DC block designed for millimeter wave applications in order to realize performance that is close to that shown in the datasheet. Alteration of the evaluation board to install a series DC blocking capacitor should be avoided as this often results in damage to the traces or at the very least discontinuities that negatively impact the RF performance. Users that choose to do so assume these risks.